Low molecular weight polyethylene waxes are frequently used as additives in coating materials such as printing inks for purposes such as an improvement of abrasion resistance, reduction of tackiness, prevention of blocking, and improvements of water resistance and water repellency of the printed film. Other uses of waxes are in hot melt compositions, such as solvent-free adhesives, as dispersants for pigments and in blends with higher molecular weight polymers. One example of a polyethylene wax is described in U.S. Pat. No. 6,858,765 and comprises an ethylene (co)polymer having a Tc of less than or equal to 0.501×(density in kg/m3)−367. The examples disclose use of significant amounts of hydrogen and dimethylsilyl (t-butylamide) (tetramethyl-cyclopentadienyl) titanium dichloride. Further the catalysts disclosed in column 6, line 48 to column 7 line 12, are bridged and do not have seven substitutions or the Cp groups are not different. Another example of a polyethylene wax is described in U.S. Pat. No. 5,723,705 in which the wax has a viscosity number of 2 to 100 cm3/g and is made using a molar ratio of olefin to hydrogen of 2 to 200.
U.S. Patent Application Publication No. 2003/0019400 discloses a polyethylene wax produced by polymerization in the presence of a metallocene compound, although the only specific metallocene compounds listed as suitable are zirconium and titanium dichlorides and the only metallocene exemplified is bis(n-butyl-cyclopentadienyl) zirconium dichloride activated with MAO. The example polymerization at 60° C. in the absence of hydrogen obtained a PE wax having an Mw of 4900 g/mol.
U.S. Patent Application Publication No. 2006/0074171, published Apr. 6, 2006, discloses polyolefin waxes, useful in hot melt compositions, having a melt viscosity at 170° C. of between 20 and 40,000 mPas. Further the only metallocene compounds exemplified are zirconium compounds and larger amounts of hydrogen appear to be used in the polymerization process.
WO 2007/061587 discloses metallocene polyethylenes having, among other things, a melt index of 1.1 to 1.6 dg/min and a density of 0.913 to 0.912 g/cc.
Hafnocenes are useful as ethylene polymerization catalysts, although generally their use has been focused on the production of high molecular weight materials. For example, U.S. Pat. No. 6,218,488 discloses a polymerization process using bridged hafnocenes to make ethylene copolymers having a density less than 0.915 and exemplifies molecular weights over 60,000, typically over 100,000.
In addition, WO 2005/103100 discloses a dual catalyst process for preparing ethylene copolymers having Mw/Mn of 6 to 100.
Moreover, U.S. Pat. No. 7,332,551 discloses polymerization of ethylene and propylene mixtures in the presence of dimethylsilylbis(indenyl)hafnium dimethyl to produce copolymers having an Mw of over 50,000.
US 2005/0282980 discloses certain hafnium catalysts used in the presence of hydrogen to make polyethylene having a melt index (190° C., 1.16 kg) of 1.4 g/10 min or less.
According to the present invention, it has now been found that certain catalyst compounds, when combined with a non-coordinating anion activator, are effective in the gas phase polymerization of ethylene to produce a novel series of high density, medium molecular weight polymers. These polymers not only exhibit attractive properties for use in traditional wax applications but also can be combined with higher molecular weight polyethylene, typically by in-reactor-blending, to produce polymer blends with enhanced physical properties.